The Japan Meteorological Agency says a volcanic eruption occurred on Sakurajima in southwestern Japan's Kagoshima Prefecture on Sunday evening.
The agency estimates that imminent massive eruptions from the volcano are unlikely.
Still, the agency has raised its alert level to the highest level of 5. It was applied at Sakurajima for the first time since the system was introduced in 2007.
Officials confirmed an eruption, just after 8 p.m. on Sunday evening, which expelled large volcanic rocks around a distance of 2.5 kilometers.
Kagoshima City officials issued an evacuation order to residents in the towns of Arimura and Furusato, with a population of 51 people in 33 households.
Meteorological Agency officials say more large volcanic rocks could fall in parts of Kagoshima City within 3 kilometers of two craters.
They also say people should be on the alert for pyroclastic flows within a radius of about 2 kilometers.
One agency official said at a press conference that "Volcanic activities of Sakurajima are becoming intense. People in residential areas should be on the highest alert for large volcanic rocks falling nearby."
Meanwhile, people in those areas have started to evacuate. A woman told NHK that she didn't hear the eruption herself and she felt a bit scared.
Two scenarios were prepared for raising the alert level to 5 at Sakurajima. One was a large-scale eruption that would affect wide areas around the volcano.
The Meteorological Agency used as a reference an eruption accompanied by earthquakes that took place in 1914, killing 58 people.
The other scenario was an eruption that is not as large, but has more volcanic activity than usual.
The agency made its prediction based on an analysis of seismicity, crustal movement and other data following Sunday's eruption. The agency is not expecting the Sakurajima volcano to cause imminent large-scale eruptions.
A task force has been set up at the prime minister's office.
Deputy Chief Cabinet Secretary Isozaki Yoshihiko told reporters on Sunday night that so far there had been no reports of damage. He called on people living near the volcano to stay informed through various media.